Surface plasmon is a quantized oscillation of free electrons that propagates along the surface of a conductor such as a metal thin film. Surface plasmon is excited to cause resonance by incident light which enters a metal thin film through a dielectric medium such as a prism at an incident angle of not less than the critical angle of the dielectric medium. This phenomenon is referred to as SPR. The incident angle of incident light that causes SPR, namely, the resonance angle, is very sensitive to changes in the refractive index of a material closest to the metal thin film. SPR sensors developed based on the above principle have been widely used for, through changes in the refractive index of a material namely a sample disposed closest to the metal thin film, the quantitative and qualitative analysis of a sample or the measurement of thickness of a sample in thin film form.
In the case where a Kretschmann configuration using a prism as illustrated in FIG. 1 is adopted to measure an angle in a SPR sensor, the angle is measured in a manner such that an incident light source of a single wavelength is focused on the center of a dielectric medium 100 and is then incident thereon in a predetermined angle range (θb˜θa), after which the light reflected from a metal thin film 300 is detected using a multichannel photodetector 400 such as a charge coupled device (CCD) or a photodiode array, in which the reflected light is emitted in the same angle range as the incident light. As such, when a resonance angle (θR) for SPR is present within the above angle range, the resonance angle at which the intensity of the reflected light is minimized and the area corresponding thereto is shown to be dark is measured in real time (U.S. Pat. No. 4,844,613). SPR sensor systems using the above method are actually commercially available from Biacore. In such a sensor system, a laser is mainly used as a light source but causes an interference problem, thus deteriorating the quality of the reflected light image. Accordingly, in a sensor system for accurate measurement of a resonance angle, hardware such as an electrical measurement unit requires high stability and precision, and a laser is thus unsuitable for use in small SPR sensor systems.
To overcome these problems, a luminescent diode (LED) having no interference may be used. However, this LED is disadvantageous because the LED light source which is not completely monochromatic but has a wavelength range of about 20˜50 nm causes the angular spectrum of SPR to be broadened, and therefore the sensitivity of the sensor is reduced, undesirably requiring the additional use of a monochromatic filter.
Further, a method of using an oscillation mirror with the use of a point detector such as a photodiode and a laser is known. This method is based on a principle in which the oscillation mirror changes the incident angle depending on time, thus measuring the reflected light intensity depending on time, and the photodiode is synchronized with the oscillation mirror, thereby obtaining a function of the reflected light intensity with respect to the incident angle (Thi js Wink, Anal. Chem. 70, 827:832, 1998). However, this method is inadequate for application to small sensors because the oscillation mirror must be accurately synchronized with the photodiode in order to measure the reflected light intensity.
Korean Patent No. 407821 discloses a plasmon resonance sensor using upconversion of active ions, but is problematic in that incident light used therefor easily incurs an interference phenomenon and the light intensity is not uniform. Also, U.S. Pat. No. 6,798,521 discloses a SPR sensor using a flat mirror, but the incident light reflected from the flat mirror has a non-uniform distribution, undesirably resulting in technical limitations in which the accuracy of results of sample measurement is decreased.